Adams et al., U.S. Pat. No. 4,712,379, for which the applicant is a co-inventor, describes a manual recycler for a combustion operated impact tool. A use can prepare the impact tool to be fired by operating the manual recycler. Operating the recycler compresses a charge of fuel air mixture behind a piston in a cylinder of the impact tool. The Adams et al. recycler includes a pump housing which a user can manually pump rearwardly against the bias of a spring before firing the impact tool. After the impact tool has been fir(ed the user can allow the pump housing to move forwardly. As the pump housing moves rearwardly a fastener engages an impart rod on the piston and carries the piston rearwardly. The piston compresses a fuel/air mixture in the cylinder in preparation for firing the impact tool. After firing, as the pump housing moves forwardly a fresh fuel/air charge is drawn into the detonation chamber and any exhaust gases remaining in the cylinder are purged through exhaust ports. In the Adams et al. impact tool the pump housing is required to move rearwardly and forwardly by about the same distance as the stroke of the piston.
While the manual recycler described by Adams et al. has proven to be practical and useful, the pump housing must be moved undesirably far to recycle the tool. This is a particular disadvantage in applications where the piston must have a longer stroke. For example, in impact nailers designed for nailing with long nails the piston stroke must at least as long as the length of the longest nails being driven. It can be tiring for a user to cycle the pump housing through a distance of several inches before every firing. There is a need for an impact tool which retains the simplicity and ruggedness of a manual recycler but requires less physical effort to use.
The prior art includes various designs for impact tools which recycle automatically. Kellerman et al. U.S. Pat. No. 5,191,861 describes one example of an impact tool having an automatic recycler. The Kellerman et al. tool has a piston which compresses an internal spring when the tool is fired. The spring returns the piston and driver to the top of the piston cylinder after each impact stroke. A disadvantage of the Kellerman et al. tool is that it requires a complicated system for accurately metering and injecting fuel gas into the detonation chamber. This system includes a relatively complicated automatic recycler which recycles the tool in several coordinated stages.
Liesse, U.S. Pat. Nos. 3,042,008 and 4,200,213 disclose nailing devices having combustion chambers defined between two pistons which face each other in the bore of a cylinder. The pistons are each mounted on springs. The provision of two pistons makes the Liesse impact tools; heavier and more complicated than is desirable.
There is an ongoing need for a simple reliable manually recycled detonation driven impact tool which can be operated with less effort than previous impact tools. There is a particular demand for such tools which have simple robust construction so that they may be manufactured in a straight forward manner and at a reasonable cost.